Field of the Invention
The invention refers to the determination of excitation profiles from excitation pulses in magnetic resonance.
Description of the Prior Art
Magnetic resonance (MR) technology (also called magnetic resonance imaging), is a well-known modality, with which images can be made of the inside of an object under examination. In simple terms, the object under examination is positioned in a magnetic resonance device in a relatively strong static, homogeneous basic magnetic field, also called B0-field, with field strengths ranging from 0.2 Tesla to 7 Tesla and more, so that nuclear spins in the subject are aligned along the basic magnetic field. In order to trigger nuclear spin resonances, radio-frequency excitation pulses (RF-pulses) are irradiated, and the triggered nuclear spin resonances are measured as so-called k-space data and MR-images are reconstructed or spectroscopic data are determined from the k-space data. For spatial coding of the measured data, the basic magnetic field is superimposed with rapidly changing magnetic gradient fields, also simply called gradients. The recorded data that are measured is digitalized and entered as complex number values in a memory organized as a k-space-matrix. From the k-space-matrix which is filled with values, it is possible to reconstruct an MR-image e.g. via a multidimensional Fourier-transformation.
Some MR-sequences use a non-selective excitation pulse, which is simultaneously applied with activated imaging gradients. Examples are the PETRA-sequence and the Silenz-sequence. The PETRA-sequence, as well as the Silenz-sequence, has the special feature of allowing for a particularly quiet imaging.
Such a non-selective excitation with activated gradients, however, has disadvantages. A specific slice selection is not possible; rather, an undesired slice selection is performed by the activated gradient at the time of the excitation, when the object that is to be measured is superimposed with the excitation profile of the pulse.
This undesired slice selection can be solved within the constraints of certain limitations, as described, e.g. in the United States Patent Application Publication No. 2013/0101198 A1. In such procedures, the exact knowledge of the actually executed excitation is important.
The excitation pulse that is used in the aforementioned sequences is usually a well-defined, rectangular-shaped pulse with an extremely short duration (e.g. 14 μs at a flip angle of 6°). Due to technical reasons, the shape of the pulse that is actually applied by the magnetic resonance device is generally slightly different from the theoretically desired rectangular-shaped pulse. The deviations from the desired shape of the pulse differ depending on the kind of magnetic resonance system and the implemented software.